The invention concerns an MAS stator for a nuclear magnetic resonance (NMR) magic angle spinning (MAS) probe head comprising a bottom bearing with at least one nozzle and at least one, in particular exactly two, radial bearings, wherein a substantially circular cylindrical MAS rotor is provided for receiving a measurement substance, wherein the MAS rotor can be supported by compressed gas in a measurement position within the MAS stator by means of a gas supply device and can be rotated about the cylinder axis of the MAS rotor by means of a pneumatic drive.
An NMR-MAS probe head of this type is disclosed in DE 10 2013 201 110 B3.
Nuclear magnetic resonance (NMR) spectroscopy is a method of instrumental analysis for determining, in particular, the chemical composition of test samples. In this connection, radio frequency (RF) pulses are irradiated into the test sample which is located in a strong, static magnetic field, and the electromagnetic reaction of the sample is measured.
In solid state NMR spectroscopy, an NMR sample is usually rotated in a tilted position at the so-called “magic angle” of approximately 54.74° with respect to the static magnetic field (“MAS”=Magic Angle Spinning) during spectroscopic measurement in order to reduce line broadening due to anisotropic interactions. To this end, the sample is filled into an MAS rotor. MAS rotors are cylindrical tubes which are closed by one or two caps, wherein the upper one is provided with wing elements (“small impellers”).
There are two variants of rotors:
1. The rotor has a blind hole. The open side is closed with the mentioned cap.
2. The rotor has a through hole. These are primarily the so-called “small systems”. They comprise the above-mentioned cap at the top and additionally a cap for below having a flat bottom (at least in case of the rotors of the applicant, i.e. the companies of the Bruker group).
The MAS rotor is arranged in an MAS stator and the MAS rotor is driven to rotate by gas pressure via the wing elements. The entity of MAS rotor and MAS stator is called an MAS turbine.
During the NMR measurement, the MAS turbine is arranged in an NMR-MAS probe head. The probe head comprises a cylindrical shielding tube (also abbreviated as “tube”) and usually a base box. The tube houses radio frequency (RF) electronic components, in particular RF resonator coils, and the MAS turbine, wherein the MAS turbine is arranged in the area of the end of the tube facing away from the base box. The shielding tube of the probe head is typically inserted from below into the vertical room temperature bore of a superconducting magnet, is positioned and held by means of hooks, supports, screws or the like. The MAS turbine is then located directly in the magnetic center of the magnet.
Exchange of an NMR probe or an MAS rotor filled with a measurement substance using a simple probe head requires removal of the probe head from the magnet, i.e. the probe head must be removed from the room temperature bore. To this end, the user kneels below the magnet, releases the holders and cable connections and collects the probe head when it slides out of the magnet. Pulling out the probe head or also reinsertion of the probe head into the magnet may require a considerable amount of force due to eddy currents induced in the metal parts of the probe head, in particular in the shielding tube, and the dead weight of the probe head. For reasons of safety, the manufacturers of probe heads require that the probe head be removed by two persons together. The rotor can then be manually exchanged on the removed probe head. A rotor change—and therefore new positioning of the probe head in the magnet—normally requires new shimming, which renders this overall procedure relatively complex.
DE 38 18 039 A1 discloses a rotatable sample magazine in the direct vicinity of the MAS stator at the probe head such that the sample in the MAS stator can be changed several times through gas pressure actuation without having to remove the probe head or the sample magazine from the interior of the magnet.
The technical poster of Shevgoor et al. “Development of a CryoMAS™ HR-MAS-MAG NMR Probe for High-field WB Magnets”, Sid Shevgoor et al., Doty Scientific, Columbia, S.C., USA, presented at ENC 2005, Providence, R.I., USA, accessible on the Internet on 9 Oct. 2008 at http://www.dotynmr.com/PDF/CryoMAS_ENC05.pdf discloses the use of a lifting system for MAS rotors. A transport line is connected at the end of the tube of a probe head facing away from the base box, which is guided through the room temperature bore of a magnet to the top and out of the magnet. An MAS rotor can be transported through the transport line into the MAS stator of the probe head mounted in the magnet using gas pressure, and an MAS rotor can also be transported in an upward direction out of the MAS stator and out of the probe head.
In order to enable quick change between various MAS rotors, thereby additionally facilitating RF shielding and keeping defined extreme temperature conditions, DE 10 2008 054 152 B3 proposes a probe head with a base box and a tube mounted to and projecting past the base box. The MAS stator for receiving an MAS rotor is disposed within the tube in the region of the end of the tube facing away from the base box. A transport line is provided for pneumatically transferring an MAS rotor within that transport line, the transport line extending in the inside of the tube from the base box to the MAS stator. However, the front bearing of this conventional arrangement does not have an opening which can be closed by means of a closure device for inserting an MAS rotor into the space between the bottom bearing and the front bearing like in a generic probe head of the above-defined type. For this reason, rotor change is not possible in the closed probe head.
In particular, MAS rotors with diameters 1.9 mm require a closure on both sides to stabilize rotation.
In order to automate rotor change, the above cited document DE 10 2013 201 110 B3 proposes a transport line for transferring an MAS rotor of an NMR-MAS probe head by means of which the rotor can be transferred into the stator. As is also disclosed in DE 10 2013 201 110 B3, the stator comprises an additional closure on the head side of the rotor in order to stabilize rotation. Problems are likely to occur, in particular, with MAS rotors having a diameter of less than 2.5 mm, since the Bernoulli force of the bottom bearing is not sufficiently reliable to maintain the rotor in position.
DE 11 2005 002 582 T5 discloses an axial Bernoulli gas bearing for an NMR MAS probe rotor (probe rotary part) driven by gas, in which an inflow having a low rotation component flows to the inside via a conically shaped rotor end. A conical through flow area is formed between this rotor end and a conical stator bearing surface. The stator presented herein is particularly suited for automated rotor change. DE 11 2005 002 582 T2 does not describe whether it is particularly suited for rotors having a small diameter. Gas discharge away from the Bernoulli bearing is also not disclosed.
U.S. Pat. No. 7,915,893 B2 discloses a stator for a CryoMAS sample, in which the measurement coils are temperature-controlled by means of a cooled He finger. The test sample can, however, be measured at a temperature around the room temperature range. This is also a pneumatic drive and for this reason, the spinner is hermetically sealed to prevent the gas supply for drive and bearing from getting into the cooled area of the measurement electronics. In order to prevent this, the gas blown into the stator is again discharged via an exhaust system. The gas discharge can be adjusted to optimize the rotation behavior of the test sample at pressures between 0.5-2 bar (see column 6, lines 32-37). It is, however, not described whether and, if applicable, how the system would be suited for rotors having a very small diameter of less than 3 mm. Rotational speeds of between 2 to 8 kHz or 300 Hz to 30 kHz are also mentioned, which indicates “large” systems. In practice, the smaller rotors are rotated at higher frequencies, wherein the limiting factor is the velocity of sound at the outer surface of the rotor. In practice, the 2.5 mm rotor is operated at approximately 35 kHz, the 1.9 mm rotor at approximately 42 kHz, and the 1.3 mm rotor at 67 kHz.
During the NMR measurement, the MAS rotor is typically supported in the stator using gas bearings in order to reduce the friction losses and thereby achieve the high rotational speeds. At least one radial bearing and one bottom bearing designed as an axial bearing are used for this purpose. Due to the high gas speed in a radial direction, the bottom bearing generates a high dynamic pressure with simultaneous reduction of the static pressure, which generates a force in an axial direction, i.e. the Bernoulli force, which holds the rotor in the stator and makes it float on an air cushion. This, however, occurs in practice only with MAS rotors having a diameter of more than 2.5 mm. This actually theoretically applies to all MAS systems. However, it does not reliably work for smaller systems. This is probably due to the small mass of the rotors and the small retention force due to the small surface of the bottom bearing such that a counter bearing is required as closure.
This problem was solved in the above-mentioned document DE 10 2013 201 110 B3 by providing a closure which can be operated via a sliding mechanism. However, this solution is suitable only to a limited extent, in particular for cooled samples, due to the material properties when the temperature changes, and is moreover quite demanding with respect to implementation and adjustment. This arrangement also requires a large amount of space which is limited, in particular, with respect to SB (“Standard Bore”). It is also obstructive when flipping into the vertical position.
In contrast thereto, it is the underlying purpose of the present invention to provide an NMR-MAS probe head of the above-defined type with stabilized rotation, which does not require the above-mentioned closure means. The system shall be automatable, i.e. allow exchange of the test sample without requiring mechanical closure at the head end of the stator/without having to remove the head from the magnet.